If you are trying to play a pre-rendered cut scene mid map, you can't.

You can play a video at the end or beginning of a map with a .cfg file.
Here are the 2 CVARs for video playing:
playvideo : cmd : : Plays a video: [width height]
playvideo_exitcommand : cmd : : Plays a video and fires and exit command when it is stopped or finishes:

I don't understand. Do I edit the chapter .cfg's and add one of those code templates? or just create a separate file? Secondly which part in one of those code templates do I enter the directory for my cut scene in?